1. Field of the Invention
This invention relates to an optical reader for use in the detection and the read of an optical mark displayed on a medium, such as a bar code reader or an image scanner and, more particularly, to an optical reader which can prevent the reading accuracy from decreasing due to normal reflection of a light on a recording surface and read a wide detecting range in a small size efficiently.
2. Description of the Prior Art
FIG. 23 is a sectional view of a conventional optical reader for use in a bar code reader, and FIG. 24 is an explanatory view showing the optical reading operation of a configuration in FIG. 23. This optical reader is of the same type as that disclosed, for example, in Japanese Utility Model Publication No. 2689/1987 (Corresponding to U.S. Pat. No. 4,143,809) or Japanese Utility Model Publication No. 11813/1986.
In this optical reader, a photoreflector 5 in which a light emitting element 2 and a photodetector 3 are integrated on a substrate 4 is contained in a case 1. An integral lens 6 is provided in the case 1. The lens 6 is formed by integrating a light emitting side lens 7 and a photodetecting side lens 8. Both the lenses 7 and 8 are of non-spherical lenses in such a manner that the optical axes a and b of the lenses 7 and 8 cross each other near a reading point X. The lenses 7 and 8 are so disposed that the centers of the light emitting element and the photodetector are located on the extension lines of the optical axes a and b.
In this optical reader, a light emitted from the light emitting element 2 is condensed by the lens 7, and focused at the reading point X. This light is reflected from a surface 10 to be detected, converged by the lens 8, and detected by the photodetector 3. On the surface 10 to be detected, the light is absorbed by black bars 11, and reflected by white spaces 12. The bar code is read by the variation in the detecting level of the reflected light.
As shown in FIGS. 23 and 24, the optical reader in which the light emitting side lens 7 and the photodetecting side lens 8 are integrally provided and the lenses 7 and 8 are aligned on the same substrate 4 can be reduced in size and is adapted for use in a bar code reader.
However, the optical reader of the above-mentioned construction has a drawback that a reading error feasibly occurs when reading the bar code by disposing the center line O substantially perpendicularly with respect to the recorded surface 10. The reason is because, when the optical reader shown in FIG. 23 is disposed generally perpendicularly with respect to the recorded surface 10, most of the light rays al of the light emitted to the surface 10 to be detected are reflected at the same reflecting angle as the incident angle .theta. of the light, and the reflected light rays bl are detected by the photodetector 3. When reading the bar code as shown in FIG. 25, the light is originally absorbed to the black bars 11, the light is reflected on the back bars 11, and the output of a photodetection signal of the photodetector 3 should exhibit waveforms as shown in FIG. 26. However, when the surface 10 to be detected on which a bar code is printed is actually enlarged, uneven waveforms as shown in FIGS. 28 and 29 are produced. The part of the light emitted to the surface 10 to be detected is reflected at random, but the other parts of the light rays of the light become, as designated by al and bl, the same incident and reflected angles (hereinafter referred to as "a regular reflection"). The regular reflection does not cause a drawback when the white space 12 is detected, but when the ratio of the regular reflection increases on the black bars 11, the light is not sufficiently absorbed to the black bars 11, reflected from the back bar 11, and the ratio of the light ray 1b detected by the photodetector 3 is sometimes increased. Specially when the quality of printing of the back bars 11 is wrong or when the unevenness of the surface 10 to be detected and the detected angle become a predetermined relationship, the regular reflection light from the black bars 11 is detected more by the photodetector 3. In this case, the level of the signal photodetected by the photodetector 3 is disordered as shown in FIG. 27. In some cases, there might be a possibility that an inversion occurs between the detecting level of the regular reflection light from the back bars 11 and the detecting level of the random reflection light from the white spaces 12. Even if the inversion does not occur, the detecting accuracy of the bar code is largely decreased.
To eliminate this drawback, in the prior invention disclosed, for example, in Japanese Patent Laid-open No. 62745/1976, an optical dispersion member 15 (FIG. 23), such as a diffraction grating or a semitransparent film is disposed in front of a lens so that the regular reflection light is not detected by the photodetector. However, since the entire quantity of the light detected by the photodetector 3 is decreased with this countermeasure, the reading efficiency is deteriorated to reduce the reading accuracy. In an OCR, in order to prevent the adverse influence of the regular reflection, the opposing angle of the light emitting element and the photodetector with respect to the recorded surface is altered. For example, the detected light is emitted sidewisely with respect to the surface to be detected or the photodetector is opposed generally perpendicularly to the recorded surface. However, this countermeasure means increases in size the reader, and has a drawback that cannot be employed as a detector, such as a bar code reader required for reduction in size.